Merge multiple compat_config.xml's.

Previously, the compat configs from static dependencies were all written
to compat/compat_config.xml, putting at the mercy of jar file merging and
so potentially losing some config.

Now the annotation processor writes a compat_config.xml for each Java
class that exports config, we we merge them together here with a new
tool.

Test: m
Change-Id: I28c12ff648a01aaad214701fca4f09743638b11b